Network Working Group M-K. Shin
Internet-Draft ETRI
Expires: November 30, 2007 T. Camilo
J. Silva
University of Coimbra
D. Kaspar
ETRI
May 29, 2007
Mobility Support in 6LoWPANdraft-shin-6lowpan-mobility-00
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Internet-Draft Mobility Support in 6LoWPAN May 20071. Introduction
A 6LoWPAN is a simple low cost communication network that allows
wireless connectivity in applications with limited power and relaxed
throughput requirements. A 6LoWPAN typically includes devices that
work together to connect the physical environment to real-world
applications, e.g., wireless sensors [I-D.ietf-6lowpan-problem].
6LoWPANs must support various topologies like mesh as well as star.
Mesh topologies imply multi-hop routing, to a desired destination.
Mesh networks are likely to consist of nodes with a certain degree of
mobility. Due to the low performance characteristics of LoWPAN
devices, mobility support should be provided without high signaling
involvement in end devices (e.g., RFD).
Fast mobility detection will be a huge challenge and LoWPAN nodes
might even change their location while being in state of hibernation.
Also, as recently seen in discussions related to MANEMO (Network
mobility for MANET), a similar point was stated regarding network
mobility in LoWPAN environments.
This document presents mobility scenarios and suggests solutions of
how to provide mobility support in 6LoWPANs.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terms Used
o Reduced-Function Device (RFD): RFDs are intended for applications
that are extremely simple, such as a light switch or a passive
infrared sensor; they can be implemented using minimal resources
and memory capacity. RFDs are not able to transmit MAC layer
beacons, and can only communicate with FFDs in a master/slave star
topology. RFDs may only associate with a single FFD at a time.
o Full-Function Device (FFD): A device implementing the complete
protocol set. FFDs have the possibility to send MAC layer beacon
frames in order to indicate their presence to other FFDs or RFDs.
FFDs can talk to RFDs or other FFDs, while an RFD can talk only to
an FFD.
o Coordinator-Function Device (CFD): A full-function device (FFD)
acting as the principal LoWPAN coordinator, configured to provide
synchronization services through the transmission of beacons. The
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Internet-Draft Mobility Support in 6LoWPAN May 2007
CFD is responsible for unique address allocation. A LoWPAN has
exactly one CFD.
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Internet-Draft Mobility Support in 6LoWPAN May 20072. Goals
Given the unique low-performance properties of LoWPANs, new
challenges arise of enabling mobility support to devices with highly
reduced memory and power. It is therefore crucial to reduce the
additional mobility related signaling overhead or to possibly avoid
it altogether. Especially to optimize power consumption, battery-
powered devices should be correctly discovered and handled by more
capable (and possibly mains-powered) devices in the network, such as
the CFD. The fundamental goals for mobility support in 6LoWPANs can
be listed as follows:
o Mobile 6LoWPAN devices must be addressable by any corresponding
node, independent of the current whereabouts.
o RFDs are not to be involved in any mobility related signaling.
o Reduction of mobility signaling messages for FFDs.
o Reuse of existing mobility protocols.
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Internet-Draft Mobility Support in 6LoWPAN May 20073. Scenario Considerations
Low-power WPAN technology is still in its early stage of development,
but the range of conceivable usage scenarios is tremendous. The
numerous possible applications of sensor networks make it obvious
that mesh topologies will be prevalent in LoWPAN environments and
mobility support will be a necessity.
Mobility based communication can also prolong the lifetime of devices
and increase the connectivity between nodes and clusters. Using
distributed LoWPANs (i.e. sensor networks), it is possible to sculpt
the devices density to cluster around areas of interest, cover large
areas, and work more efficiently by filtering local data at the node
level before it is transmitted or relayed peer-to-peer. Furthermore,
multiple controlled mobile elements can be used to provide load
balancing for gathering data. The required mobility is heavily
dependent on the individual service scenario and the LoWPAN
architecture. This document covers the following scenarios for
mobility support in 6LoWPAN.
Here are some of the key elements of an IEEE 802.16 network. Figure
1 illustrates the key elements of typical mobile 802.16 deployments.
o Device movement (e.g., FFD or RFD) within a single WPAN domain
o Device movement (e.g., FFD or RFD) between multiple WPAN domains
o Single WPAN movement (e.g., NEMO)
o MANEMO (nested NEMO)
3.1. Device Movement (e.g., FFD or RFD) within a Single WPAN Domain
Device movement within a single WPAN domain comprehends the change of
location of one LoWPAN device without losing connectivity between the
CFD/sink-node. Different behaviours can be expected depending on the
type of topology used in the LoWPAN. In star topologies, where there
is a direct communication between the RFD/FFD and the CFD/sink-node
(single-hop), the communication is not affected by the device
mobility if the mobile device stays in the radio range of the CFD/
sink-node.
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Internet-Draft Mobility Support in 6LoWPAN May 2007
+----------------------------+
| FFD |
+----+ |
|CFD | RFD |
+----+ |
| FFD |
| RFD |
| |
+----------------------------+
Figure 1: Device mobility within a single WPAN domain
In mesh topologies where the communication between the RFD/FFD and
the CFD/sink-node is multi-hop the mobility needs to be supported by
the routing protocol used within the LoWPAN. In this scenario it is
important to distinct the RFD/FFD mobility from the CFD/sink-node
mobility.
3.2. Device Movement (e.g., FFD or RFD) between Multiple WPAN Domains
In this scenario the LoWPAN devices move between different WPAN
domains as illustrated in the Fig. 2. By changing from the WPAN1
controlled by the CFD1 to the WPAN2 each device (e.g. RFD and FFD)
needs to advertise the CFD2 of its presence in order to receive new
interface configurations. Due to the LoWPAN devices characteristics
it is necessary to adjust MIPv6 behavior to such networks. Moreover
it is important to understand that RFD represents several limitations
regarding FFD, meaning that each one will have different roles
regarding the handover process.
+-----------------+ +------------------+
| FFD | | FFD |
| +----+ +----+ |
| FFD |CFD1| |CFD2| RFD |
| +----+ +----+ |
| | | |
| RFD >------> RFD |
| FFD >------> FFD |
+-----------------+ +------------------+
Figure 2: Device mobility between multiple WPAN domains(1)
Another consideration should be made if the moving LoWPAN device is
the CFD1. In this case, such device will act as a CFD until he finds
another CFD responsible for a new domain. Fig. 3 illustrates the
situation when the sink-node (CFD1) moves from his domain to another
domain becoming a common FFD, after negotiating with CFD2. The
former domain of CFD1 will need to elect a new CFD, in the example
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Internet-Draft Mobility Support in 6LoWPAN May 2007
the CFD3.
+-----------------+ +------------------+
| FFD | | FFD |
| +----+ | +----+ |
| |CFD3| | |CFD2| RFD |
| +----+ | +----+ |
| +----+ | |
| RFD |CFD1|------> FFD |
| +----+ | |
+-----------------+ +------------------+
Figure 3: Device mobility between multiple WPAN domains(2)
3.3. Single WPAN Movement (e.g., NEMO)
In this scenario we consider the aggregation of nodes (FFDs and RFDs)
in clusters, with the introduction of an elected node that will work
as a Coordinator Function Device (CFD). This node will be
responsible for the management of the cluster communication with the
external networks.
+----------------------------+
| FFD |
+----------+ +----+ |
| External |--- |CFD | RFD |
| Network | +----+ |
+----------+ | FFD |
| FFD |
| |
+----------------------------+
Figure 4: WPAN mobility
In this section the WPAN is considered indivisible and presents
mobility as an entity. Moreover, we consider a mobile WPAN as a leaf
network, as it does not carry transit traffic.
The CFD must be a FFD, as it requires supplementary functionalities
when compared with RFDs, as extra processing and energy power. None
of the FFDs and RFDs behind the CFD need to be aware of the WPAN
mobility, being its movement completely transparent to those devices
inside the mobile WPAN. The protocols that support the WPAN movement
are very dependent of the following issues:
- Application requirements
- Level of mobility of WPAN
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Internet-Draft Mobility Support in 6LoWPAN May 2007
The level of mobility of the WPAN requires different routing updates.
It is crucial that the routing protocol optimize the traffic routes
as the energy consumption is critical in node forwarding. Using the
CFD concept is possible to create an architecture that reduces the
energy consumed in WPAN movement, and thus increasing performance in
these networks.
The IPv6 [RFC2460] supports natively the mobility. Moreover, in
contrast with IPv4, IPv6 offers extra functionalities for router
optimization. However, it presents overheads and the routing is not
optimal in the case of network mobility. The Nemo working group
[RFC3963] that studies these scenarios, has already proposed a simple
solution to transparently solve, part of the present needs. However
the applicability to LowPAN networks is not trivial and requires
extra adaptability to its limited characteristics. The new
challenges in providing mobility for WPANs include resource
management, network coverage, network lifetime, topology change,
routing protocols, security, data reliability, QoS and timely
dissemination. These challenges affect the performance of the mobile
WPANs.
3.4. MANEMO - Nested NEMO
In these scenarios, terminal devices or WPANs inside a WPAN could
present mobility support, travelling to other fixed or mobile WPANs.
These entities can move inside or outside high-level WPANs, forming
nested entities. Sink node mobility, as a unique entity, was
presented as a special case in sections 3.1 and 3.2.
+----------------------------+
| FFD |
+----------+ +----+ |
| Exterior |--- |CFD | RFD |
+----------+ +----+ |
| FFD |
| FFD +--------+ |
| | WPAN | |
+------+ | +--------+ |
| WPAN | | RFD |
+------+ +----------------------------+
Figure 5: MANEMO mobility
In these scenarios, due to its properties, several issues need to be
covered (i.e. route optimization, bandwidth and encapsulation
overhead).
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Internet-Draft Mobility Support in 6LoWPAN May 2007
In nested networks - in more complex scenarios, the problems are more
accentuated and need several improvements and adaptations. Even if
we use the redirect IPv6 properties [RFC3775], there stills to exist
indirection and overhead, which is more critical when there are
several hierarchical levels.
Among the open research problems, real time solutions that result in
low mobile device speeds and cooperation between multiple mobile
devices stand out as challenges that have significant impact.
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Internet-Draft Mobility Support in 6LoWPAN May 20074. Mobility Support in 6LoWPAN
In this section, some solutions and optimization techniques for each
scenario described in section 3 will be described.
4.1. Device Movement (e.g., FFD or RFD) within a Single WPAN Domain
In this scenario, there is no need to define mobility protocols
additionally. The mobility can be supported by the routing protocol
used within the 6LoWPAN. The first way to achieve this goal is to
re-use existing MANET protocols without making any modifications.
(e.g., AODV, OLSR, DYMO, Etc.)
However, the modification or simplification of existing MANET routing
protocols may be required for dynamic routing to be feasible in a
LoWPAN domain, because other requirements apply to LoWPAN devices.
Unlike MANET devices, LoWPAN nodes are characterized by much lower
power supplies, smaller memory sizes, and lower processing power,
which create new challenges on obtaining robust and reliable dynamic
routing within LoWPANs.
There exists a trade-off relationship between routing effectiveness
and the requirements posed upon the devices participating in a
dynamic network. The challenge is to create a balance between
protocol simplicity and routing performance. But stripping down
existing protocols to power-aware, low-overhead protocols decreases
the efficacy and functionality of their sophisticated routing
techniques, or possibly even endangers the goals they were designed
for. The issues is being discussed now in [I-D.dokaspar-6lowpan-
routreq].
The other way is to develop a new routing protocol for 6LoWPAN. The
work is also being discussed in [I-D.culler-rsn-routing-reqs] and is
especially focused on sensor networks. Considering the variety of
sensor based applications, there may not be a single routing protocol
satisfying the entire list of requirements, in which case it may be
decided to define a limited set of routing protocols that could be
combined to satisfy the overall objective.
4.2. Device Movement (e.g., FFD or RFD) between Multiple WPAN DomainsS
In this scenario, MIPv6 [RFC3775] could be considered for mobility
solutions. However, as listed in goals, RFDs should not to be
involved in any MIPv6 mobility related signaling and mobility
signaling messages in FFD should be reduced if possible.
To support efficiently this scenario, network-based mobility
management approach (e.g., Proxy MIPv6 [I-D.ietf-netlmm-proxymip6])
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Internet-Draft Mobility Support in 6LoWPAN May 2007
would be preferred.
The goals of network-based mobility management approach [RFC 4831]
are:
o Handover Performance Improvement
o Reduction in Handover-Related Signaling Volume
o Location Privacy
o Limit Overhead in the Network
o Simplify Mobile Node Mobility Management Security by Deriving from
IP Network Access and/or IP Movement Detection Security
o Link Technology Agnostic
o Support for Unmodified Mobile Nodes
o Reuse of Existing Protocols Where Sensible
o Localized Mobility Management Independent of Global Mobility
Management
o Configurable Data Plane Forwarding between Local Mobility Anchor
and Mobile Access Gateway
Almost of the goal above fits to our goals for mobility support in
6LoWPAN described in section 2.
Host-based mobility protocols (e.g., MIPv6) require a number of
periodic signaling messages (e.g, Binding Update in MIPv6) at end
devices. It can increase power consumption. In network-based
mobility protocols does not require any mobility protocols in end
devices. Instead, gateway (CFD) performs mobility functions (e.g.,
Proxy BU).
At this phase, current PIMv6 defines the device-to-gateway interface
applied in a single-hop. However, in this scenario, multi-hop and
mesh topologies should be considered. So, to use PMIPv6 in 6LoWPAN,
the interface for multi-hop and mesh topologies between devices and
gateway (CFD) should be extended and defined (e.g., ad-hoc manner
support).
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Internet-Draft Mobility Support in 6LoWPAN May 20074.3. Single WPAN movement (e.g., NEMO)
This scenario is exactly the same as that of NEMO. NEMO support
[I-D.ietf-nemo-requirements] is concerned with managing the mobility
of an entire network, viewed as a single unit, which changes its
point of attachment to the Internet and thus its reachability in the
Internet topology. Such a network is referred to as a mobile network
and includes one or more mobile routers (MRs) which connect it to the
global Internet. So, in this scenario a mobility network and MRs are
mapped into WPAN and CFDs, respectively.
To support this scenario, basic NEMO support protocol [RFC3963]
should be supported in 6LoWPAN.
4.4. MANEMO - Nested NEMO
MANEMO is a special case for Nested NEMO. When mobile routers (CFDs)
and mobile nodes (RFDs/FFDs) converge at the edge of the Internet
using wireless interfaces, they can form a 6LoWPAN network in an ad-
hoc fashion and are able to provide Internet connectivity to one
another. Several issues exist in this network configuration such as
network loop, un-optimized path and multiple exit routers to the
Internet. They are well-known MANEMO' issues. While fixed routers
provide constantly connectivity, mobile routers (CFDs) can experience
intermittent connectivity to the Internet due to their movement.
When NEMO Basic Support [RFC3963] is used in this context, network
loops naturally occur. So, a new MANEMO solution is required in
6LoWPAN.
MANEMO solution is not finalized yet and it is at initial stage. If
it is done, to support this scenario, it should be also supported in
6LoWPAN.
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Internet-Draft Mobility Support in 6LoWPAN May 20076. Security Considerations
RFD nodes must have a means of identifying friendly nodes and
distinguishing them from not trusted nodes. Especially if the RFDs'
mobility support is handled by an FFD or CFD, there must be some way
for the RFD to tell whether that more capable device can be trusted
or not.
More to be defined.
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Internet-Draft Mobility Support in 6LoWPAN May 2007
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